This proposal will determine whether cell cycle position and phosphorylation are critical determinants for activation of latent p53, and will measure the activation of its transcriptional targets. The Specific Aim include 1) addressing the physiologic relevance by measurement of phosphorylation during the cell cycle, determination of DNA damaging agents that activate p53 via phosphorylation, and determination of which targets are activated by phosphorylated p 53, 2) study of the effect of C-terminal phosphorylation and intramolecular interactions on modulation of p53 activity, 3) addressing the mechanism of transactivation by measurement of binding affinities to natural p53- binding sites on DNA and the effect of adjacent sequences and binding proteins, and 4) discrimination of cellular conditions that distinguish p53 two effect of growth arrest and apoptosis.